Marcin Pilipczuk, Magnus Wahlström. W[1]-hard, even for four terminal pairs. In Robert Krauthgamer, editor, Proceedings of the Twenty-Seventh Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2016, Arlington, VA, USA, January 10-12, 2016. pages 1167-1178, SIAM, 2016. [doi]
Abstract is missing.